In recent years and continuing, optical discs (e.g., CDs (Compact Disc) and DVDs (Digital Versatile Disc)) serving to record computer programs, audio information, video information (hereinafter also referred to as “contents”) are drawing greater attention due to the advances in digital technology and the improvements in data compression technology. Accordingly, as the optical discs become less expensive, optical disc apparatuses for reproducing the information recorded in the optical discs have grown to become widely used.
The amount of information to be recorded in the optical discs is growing year by year. Therefore, further increase in the recording capacity of a single optical disc is expected. As for measures that are being developed for increasing the recording capacity of the optical disc, there is, for example, increasing the number of recording layers. Accordingly, vigorous research is being made on optical discs having plural recording layers (hereinafter referred to as “multilayer disc”) and optical disc apparatuses that access the multilayered discs.
In the multilayer discs, there is a possibility that the signals from a target recording layer be adversely affected by spherical aberration if the spaces between the recording layers are too large. Accordingly, there is a trend of reducing the space between the recording layers. However, reducing the space between the recording layers causes cross-talk between the recording layers (so-called “interlayer cross-talk”). As a result, the bundle of rays returning (reflected) from the multilayer disc contains not only desired beams reflected from a target recording layer (hereinafter referred to as “signal beams”) but also a significant amount of undesired beams reflected from recording layers besides the target recording layer (hereinafter referred to as “stray beams”). This leads to the decrease in S/N ratio of reproduction signals.
An apparatus which reduces interlayer cross-talk during reproduction of a multilayer disc is proposed in, for example, Japanese Registered Patent No. 2624255.
However, the apparatus disclosed in Japanese Registered Patent No. 2624255 requires the diameter of a pinhole to be further reduced in order to further reduce stray beams incident on a detector. The reduction of incident stray beams, however, leads to undesired reduction of signal beams incident on the detector.